In many wireless communication systems there exist a system model where a source node communicates with a destination node via the assistance of one or multiple relay nodes. In an environment where there are multiple relays available, one can select a single relay from all the available relays to transmit the signal. Such a technique, referred to relay selection, has been shown to effectively achieve the diversity order offered by using all the available relays. In some circumstances, the channel characteristics from the source to the relays and from the relays to the destinations can be quite different.
In practical dual hop systems with relay selection, there are different protocols that the relays can follow to assist the transmission from the source to the destination. The most popular ones are amplify-and-forward and decode-and-forward. For amplify-and-forward, the relay nodes receive the signal from the source, amplify it with some gain (either a fixed value or a variable value according to instantaneous channel gain), and then forward the amplified signal to the destination. For decode-and-forward, the relay nodes detect the received signal from the source, and then forward the detected signal to the destination. Both protocols have their pros and cons. For example, decode-and-forward results in a higher complexity relay node compared to amplify-and-forward, but can usually provide a better performance.
There are different modes of operation for selecting the relay to transmit the signal from the source to the destination. Most of the current systems either use a first mode i.e. forward selection; where selection is made based on instantaneous channel characteristics between the source and relay, or a second mode i.e. backward selection; where selection is made based on instantaneous channel characteristics between the relay and the destination. Whether forward selection or backward selection is used is decided at the beginning of system initialization, based on the channel conditions at the time of selection. Existing systems do not take into consideration changes to the channel conditions that can occur during a transmission. Once the selection scheme is decided at system initialization, it is fixed all through the transmission, regardless of the changing channel conditions. In other words, in most of the current systems, when it is decided to perform forward selection, will continue so without considering the varying channel statistics even if other modes of operation favours the transmission when channel conditions change.
There is therefore a desire for an intelligent system that is able to adapt the relay selection schemes according to the variation of the channel characteristics.